After 30 years of friendship but surprisingly no prior collaborations, film icons Martin Scorsese and Jack Nicholson have finally teamed up for a cat-and-mouse crime thriller, "The Departed."

The film, which thrusts the audience into a gritty battle between Boston police and the Irish-American mob, stars Leonardo DiCaprio as Billy Costigan, a cop who goes undercover in an attempt to gather information about a crime ring run by volatile mob boss Frank Costello (Nicholson). Matt Damon plays Colin Sullivan, an ambitious, corrupt officer who has infiltrated the elite Special Investigation Unit to help Costello stay a step ahead of the law. Things boil over when both sides discover they're being betrayed by a mole, and the two men must race to expose the other before being caught.

It's a fiery, tightly wound plot — and the film's set was unsurprisingly equally intense. "Jack Nicholson in character is so entirely unpredictable you almost expect the unexpected in a lot of ways," said DiCaprio.

How unpredictable?

While shooting one scene, "the prop guy sort of tipped me off that he had a gun and a fire extinguisher and a box of matches and whiskey under the table, and he had no idea what Jack was gonna do," DiCaprio recalled. "Nicholson ended up pulling a gun on my face and lighting the table on fire."

Nicholson also flaunts some eccentric bravado by handling a disembodied limb, eating an insect, appearing inexplicably covered in blood, and, of course, cavorting with women. Despite buzz about the controversial, Nicholson-engineered scene involving his character, two ladies, a pile of cocaine and a certain sex toy, the final cut of "The Departed" leaves much of that scenario to the imagination.

The film never shies away from exploring the stresses of living a double life, though. "It was one of the most compelling characters I've ever had to play," DiCaprio said of his third collaboration with the "Gangs of New York" and "The Aviator" director. Costigan desperately wants out of the mob, but dutifully remains undercover to bring Costello down. "It was difficult because I didn't know how much to convey to the audience about what I was going through. But at the same time, realizing I was in a room filled with killers, how much do you let on that you're petrified?"

Damon's Sullivan seems more at ease with his double identity. In the midst of the drama, he manages to smoothly handle an unsuspecting girlfriend and land a job promotion while continuing to tip off Costello. However, he noted, "All the violence in this film, a lot of graphic, brutal violence — it's also shown as coming with a price."

The character who generates the most fear and violence is clearly Costello, whose madness grows more apparent as the plot progresses. "He has all the power. But now, he starts taking risks, like putting himself on the front lines of drug deals," said Scorsese. "He knows he doesn't have to do that; it's just for the thrill of it at this point in his life. Costello has gotten too old, and he knows, ultimately, he is probably on his way out. It was interesting to watch Jack portray Costello starting to unravel."

Martin Sheen, Mark Wahlberg and Alec Baldwin also star in the film, along with up-and-comer Vera Farmiga ("The Manchurian Candidate"). The screenplay was adapted from a successful Hong Kong script by "Kingdom of Heaven" writer William Monahan, though the U.S. version's actors were quick to point out that the film isn't a straight-up remake of 2002's "Infernal Affairs," which was directed by Wai Keung Lau and Siu Fai Mak.

"It's a very kinda Boston sense of humor, very dark humor," Damon said of Monahan's dialogue. Wahlberg and Damon are both Beantown natives. "The characters are in so much danger for so many parts of the movie that to be able to break it up with a laugh every once in a while was a relief."

"I was a huge fan of ['Infernal Affairs'] after I watched it, but as good as that film was, it's hard to call any movie Martin Scorsese makes a remake," said DiCaprio. Damon agreed, "This movie is so unmistakably about Boston. I loved and appreciated the original film but this became about the subculture of the Boston police where I'm from, so we went off in our own direction."